1. Field of the Invention
This invention relates to particular water-soluble polymer compositions containing polyacrylic acid and copolymer of itaconic acid and acrylamide and their use as combined dispersant and fluid loss control additives for aqueous drilling fluids which are used in drilling wells in subterranean formations.
2. Description of the Prior Art
Rotary systems are generally used in drilling oil and gas wells. These systems depend upon rotation of a string of drill pipe to the bottom of which is attached a drill bit. The bit cuts into the formation causing the cuttings to accumulate as drilling continues. A drilling fluid is used which lubricates the drill bit and carries cuttings to the surface for removal. This allows the bit to continue functioning and the bottom hole to be clean and free of cuttings. The drilling fluid is also the source of pressure which is exerted against the formation. Even in other drilling systems, drilling fluids are still needed to remove bore hole cuttings and to otherwise perform functions related to drilling fluids.
Control of rheology of the drilling fluids, i.e., plastic viscosity, yield point and gel strength is needed to assure effective pumping of the drilling fluid. Usually this means reducing yield point and gel strength of the drilling fluid. One means for control is the addition of thinners (dispersants) to the drilling fluid. These materials are generally anionic in charge and are attracted to the electrically charged clay particles. Through neutralization of the charge on the clay particles by the thinners or dispersants, the rheology of the drilling fluid can be controlled for a more efficient drilling of the well. Examples of thinners or dispersants are lignites and lignosulfonates such as chrome lignosulfonate and ferrochrome lignosulfonate. However, the chrome and ferrochrome lignosulfonates have the disadvantage of containing heavy metals which cause environmental problems and are not as effective in seawater.
The homopolymer of acrylic acid has also been used as a thinner.
Further, oil and gas producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. When drilling through such porous formations, it is essential to employ drilling fluids having characteristics such that excessive amounts of the drilling fluid do not penetrate the porous formation. Drilling fluids have a tendency to penetrate the formation because pressure in the bore hole is greater than the pressure opposing it which comes from the formation. Should excess penetration occur, there is loss of drilling fluid into the formation resulting in loss of pressure, inability to drill and remove cuttings and in the case of loss from brine muds, extra expense because of their cost.